Application of amino resin crosslinker
The main function of amino resin (melamine formaldehyde, benzaldehyde and urea formaldehyde) in thermosetting coating is to crosslink the main film-forming material molecules into a three-dimensional (three-dimensional) network structure through chemical reaction. This kind of network structure is obtained by the reaction of amino resin molecules with the functional groups on the film-forming materials and the polycondensation reaction with other amino resin molecules at the same time. Amino resin is easy to react with polymers with primary and secondary hydroxyl, carboxyl and amide groups, so amino resin is usually used in paint systems based on acrylic, polyester, alkyd or epoxy resin.
Amino resin is also used in polyurethane system as an additive to improve the comprehensive properties of coatings for some applications.
Principle of amino resin:
The importance of amino resin in baking paint is far more than its proportion in paint. It is more and more important to know how to use the chemical properties of amino resin to design the coating formulation. For example, if the coating formula designer is not satisfied with some properties of the coating, he can adjust it by the following methods:
1. Improvement or reselection of film-forming resin itself;
2. The choice of amino resin (methyl ether or butyl etherification, and the choice of etherification degree, etc.);
3. The matching ratio of film-forming resin and amino resin.
4. Choice of catalyst (add or not, or how much.
All of the above 4 items are related to amino resin except the first one, and the performance of amino resin depends on its functional group and activity, so it is important to understand the structure of amino resin. But before we understand the amino resin, we should have a preliminary understanding of the main resin matching with the amino resin.
The amino resin mentioned above is mainly used with alkyd resin, acrylic resin, polyester resin and epoxy resin. Alkyd resin is mainly synthesized by esterification of polyol and polyacid resin. In the process of synthesis, the general alcohols will be in proper excess; there will also be some carboxyl groups of polyacid that do not react completely, so the final alkyd resin will contain a certain amount of carboxyl and hydroxyl groups. The number of carboxyl and hydroxyl groups is usually characterized by acid value and hydroxyl value. Acid value refers to the milligram of KOH required for neutralization of 1G solid resin by KOH titration. Hydroxyl value refers to the milligram number of KOH required for the complete titration and neutralization of 1G solid resin containing OH converted into carboxyl group. Similarly, polyester resin, acrylic resin and amino resin also contain certain carboxyl and hydroxyl groups. However, the raw materials used for the synthesis of resin are different. For example, the carboxyl group in acrylic resin comes from acrylic acid, the hydroxyl group comes from hydroxy acrylic acid, and the amount of carboxyl group and hydroxyl group in amino resin is also different. Acid value, hydroxyl value and viscosity are all important indexes of resin, which directly affect the performance of resin.
Back to the topic of amino resin, let's first look at the structure of amino resin:
Figure 1,
Figure two,
Fig. 1 is a partially alkylated amino resin containing alkoxy, imino and hydroxymethyl groups. If the six membered ring composed of carbon and nitrogen atoms is regarded as a skeleton, the frame or branch derived from it can be described as three heads and six arms. The variety of properties of amino resin is formed by the difference of these six "arms" and their intricate arrangement.
Figure 2 shows an extremely symmetrical structure of Hmmm, i.e. all methylated amino resin, with only one functional group: methoxy, which is ideal. Due to the fact that the etherification degree can not reach 1:6 (maximum) in actual production, there is always a little presence of imino and hydroxymethyl in the said all methylated amino resin.
Let's start with the principle of amino resin to understand its properties:
The first step of resin synthesis is to make melamine react with formaldehyde in the presence of catalyst to form POLYHYDROXYMETHYLMELAMINE. All the active hydrogen atoms in the triazine ring can be converted into hydroxymethyl, but in fact, two to six moles of formaldehyde react on the triazine ring, and those remaining unreacted active hydrogen atoms are represented by imines. We will see in the future that these groups play an important role in the curing process through self polycondensation.
POLYHYDROXYMETHYLMELAMINE is very unstable and has limited solubility in conventional coating solvents. Amino resin mainly plays a role of crosslinking and curing in coatings. In order to make a crosslinking agent suitable for coatings, it is generally necessary to etherify hydroxymethyl with a short chain alcohol to reduce its reaction activity and improve its solubility with conventional film-forming materials and aliphatic solvents. Short chain alcohols generally use methanol and butanol. By controlling the amount of methanol or butanol and other conditions, amino resins with different degrees of etherification can be obtained.
The unreacted hydrogen atom (imino) does not react with short chain alcohols. In addition, this reaction shows that all six hydroxymethyl groups react with alcohol to form hexaalkoxymethylmelamine, which can actually control one to six hydroxymethyl groups to react with alcohol. So there are so many different kinds of amino resins.
Self polymerization of amino resin:
The molecular weight of amino resin is determined by the degree of self condensation or bridging between the functional groups (imino, hydroxymethyl, alkoxymethyl) on triazine ring and melamine. In the final application, the molecular weight of amino resin affected by the degree of bridging polymerization has a great influence on the performance of the film.
The self polycondensation of amino resin can take place in the following ways:
Figure three,
Among them, the reaction marked on the left side generates methylene bridge, and the reaction on the right side generates methylene ether bridge. The degree of bridging of amino resin is usually expressed by degree of polymerization (DP): DP = molecular weight / weight of each triazine ring. The amino resin produced in the early stage is self polymerization type, DP > 3.0. With the development of technology, it is possible to reduce the self polycondensation of amino resin products to the minimum. At present, as low as DP = 1.1 in the commercial cyanuric acid resin.
The main effect of amino resin molecular weight can be reflected in the viscosity of coating. The melamine resin with DP > 2.0 must be released to 50% - 80% solid with solvent olefin, so as to achieve the applicable viscosity. Melamine resin with monomer DP between 1.1 and 1.5 can usually be supplied in 100% effective solid form, and the extra solvent has a great influence on VOC of the finished coating. The molecular weight of amino resin also affects the curing reaction and coating performance. A paint system with high DP amino resin will take a shorter time to reach the specified crosslinking density than a paint system with the same structure but lower DP amino resin, so the paint with high DP crosslinking agent only needs less catalyst or weaker acid catalyst to reach the same curing state. The effect of molecular weight on the film properties is mainly in the range of flexibility. The film cured with high DP amino resin contains higher percentage of amino amino bond and less amino paint bond. This type of crosslinking network structure forms a coating with good hardness, but may be brittle. Sometimes it can be compensated by choosing a more flexible paint resin. However, it is generally required that the application of high toughness coating needs the amino resin of monomer.
Polyester containing carboxyl group may react with melamine formaldehyde to produce useful thermosetting surface coating, which has a wide range of physical properties.
Many butylated melamine formaldehyde resins have commercial interests. First, the initial polymerization degree (molecular weight) is different, and the proportion of alkoxy group to hydroxymethyl group and amino hydrogen free is different. These differences will affect the viscosity of liquid, compatibility of melamine with polyester and curing speed of enamel. Because the traditional melamine resin reacts with the side hydroxyl group, it mainly crosslinks with the polyester molecule. Because the crosslinking reaction is acid catalyzed, when the curing temperature is between 120 ℃ and 150 ℃, generally polyester resin will affect the crosslinking reaction in strong acid, however, some polyester in extremely weak acid, need to add acid catalysis to let the enamel system cure.
Coating properties
Formaldehyde content
Types of alcohols
Alcohol content
High (less imino)
Low (imino poly)
Long (e.g. butanol)
Short (e.g. methanol)
Large (less hydroxymethyl)
Small (hydroxymethyl poly)
viscosity
low
high
Low medium
low
low
high
Hardening rate
slow
fast
slow
Very fast
slow
fast
Compatibility
Very nice
difference
Very nice
difference
Excellent
difference
stability
Extremely high
difference
good
difference
good
difference
MST
Very nice
difference
Very nice
difference
Extremely high
difference
There are the following phenomena: in addition to the cross-linking reaction of melamine polyester, butylated melamine formaldehyde resin also undergoes self polycondensation reaction. In other words, amino resin self crosslinks to form melamine network structure. This reaction occurs simultaneously with melamine polyester reaction and is a competitive reaction. The reason for the reaction is that the melamine formaldehyde resin containing butyloxy group, free hydrocarbon methyl group and imino hydrogen, all of which can react with each other. Once the amino resin self crosslinks, it will lose some functions.
Although self crosslinking often makes the coating have greater hardness and chemical resistance, but the loss of elasticity is very large. In order to obtain enough elasticity of polyester paint.
Hexamethoxymethylmelamine (hmmm) is a kind of monomeric amino resin with complete hydroxymethylation and complete methylation. Similar to butylated melamine formaldehyde, it crosslinks with the hydroxyl group of polyester resin when heated to form a non softened solid. In essence, the self crosslinking of hmmm will not occur even if the time is longer or the temperature is higher without the action of acid catalyst. However, the bulk hmmm will self crosslink at 150 ℃ with strong acid catalyst. On the contrary, even when there is no strong acid, the traditional melamine and urea resins will have strong self crosslinking reaction with the increase of temperature.
Curing reaction of amino resin:
Because amino resin is used to cross link the main film-forming material molecules into a network structure, it is interesting to see the polycondensation reaction between amino resin and paint resin. A typical example is the ether (exchange) reaction of hydroxyl group on paint resin and alkoxymethyl group on amino resin: as shown in the figure below.
Fig. four:
From the above figure, you can imagine how melamine molecules (marked with m in the figure) are pulled together with hydroxyl groups from different film-forming polymers in the micro world to form a three-dimensional network structure, which determines the performance of the paint film.
In the presence of heat and acid catalysts (usually curing conditions), the crosslinking reaction takes place quickly, connecting all available hydroxyl groups on the paint. In fact, when the polymer network structure is formed, the fluidity of reactants is decreasing, and some hydroxyl groups remain unreacted. In general, when there is more amino resin than the ideal ratio in the coating, the remaining alkoxy group can participate in other reactions or remain in the coating without reaction. As mentioned earlier, amino resin is easy to self crosslink and react with each other, resulting in the increase of molecular weight in production. These reactions also occur when the film is cured. In this way, it is not so much a negative factor as a certain degree of self crosslinking of amino resin, but an essential factor to obtain a good durability and compact polymer matrix. Amino resin all